AND THE DIVER BECOMES A SWIMMER

We describe and model experimental results on the dynamics of a ”ludion” - a neutrally buoyant body - immersed

in a layer of stably stratified salt water. By varying periodically the hydrostatic pressure, the ludion

initially positioned at its equilibrium height oscillates vertically. Depending on the ratio of the forcing frequency to

the buyancy frequency N of the stratified fluid, the ludion can emit its own internal gravity waves that we

measure by Particle Image Velocimetry. Our experimental results describe first the resonance of the vertical

motions of the ludion when excited at different frequencies taking

into account added mass and added friction coefficients (Voisin, 2007). Then, for the larger oscillation amplitudes,

we observe and describe a bifurcation towardsfree horizontal swimming with chaotic trajectories.

The statistical analysis suggests the existence of an underlying non equilibrium thermodynamics with even possible

”condensations” of ludions in pairs or in triplets when several

ludions are introduced in the container. Finally, we also observed that the ludion can interact with its own internal

gravity wave field and possibly become trapped in the container gravity eigenmodes. However, it seems that,

contrary to the surface waves associated with Couder walkers (Couder et al., 2005) the internal waves are

not the principal cause of the horizontal swimming (Deng et Caulfield, 2018). This does not however, exclude

possible hydrodynamic quantum analogies to be explored in the future (Couder et al., 2005, Bush, 2015).